Audio frequency transmission circuits



Oct. 24, 1933. s vo 1,931,664

AUDIO FREQUENCY TRANSMISSION CIRCUITS Filed Aug. 21, 1930 A ud/a 04/730117 Had/o Oufpuf' amnion Aud/o Fieyuency Patented Oct. 24, 1933 AUDIO FREQUENCY TRANSMISSION CIRCUITS Stephen D. Lavoie, Winthrop, Mass," assignor,

by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application August 21, 1930. Serial No. 476,847

12 Claims. (Cl. 179-171) the following specification taken with the ac- This invention relates to audio frequency transmission circuits and more particularly, to a tone control; for regulating the width of the band. of frequencies which may be transmitted.

In the electrical transmission of audio frequency signals, for example speech or music, it is evident that all frequencies within the useful audio range should be transmitted with equal gain, if the audio output is-to be a faithful reproduction of the audio input. Within recent years considerable Work has been done upon the design of audio frequency transformers and audion amplifier circuits, in an effort to secure a substantially uniform gain for all frequencies between -50 or 100 cycles and approximately 5000 cycles. Theaudio systems included in many commercial radioreceivers are'of thistype and provide for an approximately faithful reproduce tion of a wide range of audio frequencies. In the absence; of local electrical disturbances, which give rise to static noises, audio reproduction of good quality is obtained when receiving strong signals, for example signals from a local.

broadcasting station.- When weaksignals are received, however, the increased radio frequency amplification required to secure the desired audio output result in theappearance of static disturbances, or of beat notes'due to the heterodyning with an, adjacent carrierfrequency which, in part at least, mask the signal which is being received. f

Fortunately, however, these disturbances which frequently appear when weak signals are ,re-

ceived are, in generalgiof-relatively high fre-'- quency and they maybe reduced or eliminated by shifting the upper limit of the band of transmitted frequencies to a lower value. Although this lowering'of'the cutoff results in some distortion of the desired signal, the net effect is a material improvement in the quality'ofthe audio reproduction since the disturbing noises maybe reduced or completelyleliminated.

, Anobject'of the present inventionjis to provide andimproyedj tone Golly/11012 101 use in an} audio frequencyjtransmission line, and particularly in anjaudio frequency amplifier employing vacuumtubes. "A further object offthe invention'is to provide a tone control of thetype stated which includes adjustable; elements for alteringeither the PQSi tion or the slope, or both the position and slope,v of the upper limit of-the transmission-frequency characteristic curve.

. These and other objectswillbe apparent from companying diagrams in which, v t

Fig. 1 is a circuit diagram illustrating the invention as applied to an audion amplifier, and

Figs. 2 and 3 are curves" of audio output against audio frequency showingresults obtained by use of the invention.

Referring to Fig. 1, the inductance L is in serted between an audio'frequency amplifying tube 1 and its output transformer 2. The value of the inductance L is chosen to choke out, all impulses above apredetermined high frequency. To control the point at which the high frequency impulses are cut out, the inductance L is by-passed by a line 3 which includes a variable resistance R. When the resistance R is all out the inductance L 'is ineffective. However, by increasing R, more current is forced through the inductance L with a consequent choking out of'the'high, undesirable frequencies. This resistance has a'maximum impedance considerably higher than that "of the inductance L, sov that when the effective resistance reaches it maximum' value, the inductance L will alone determine what high frequencies 'are passed.

' The result accomplished b'y'the inventionas described so far, is shown lby curves in Fig.-2; The curve A represents the'values of audioout put to audio frequency with the resista'nceR all out and the inductance L consequently Icy-passed 'by the line 3. As the resistanceR is put in the circuit the higher frequencies are choked out by the inductance Land the transmission-frequem cy 'curve takes the form B. It can clearly be seen, therefore, that there will be no reception of impulses'having higher frequencies than those determined by the curve B.

At times, it is desirable that the high frequency cutoff should not be sharp, as shownby curvesA and B. Such a case arises when a particular loud speaker exhibits peaks atsome fre-' quencies. Also it is sometimes desirable" to let through a few overtones even though accompa nied by a sight disturbance. A resistance R1 con nected in series with theinductanceL has the effect'of changing the shape-of the curve. "This resistance is variable so that any desired shape? may be given to the curve at cutoff. Such fiat--' tened curves .are C and D as shown in Fig. 2:

Incidental to cutting ofthe high frequencies by theinductance'L and 'controllingresistance R,-there is a loss ,in;output volume. In order to compensate for this loss, a gain-control resistanceRz is shunted across the transformer 2,-to regulate that portion of the load developed the audio output to audio frequency when the inductance L is bypassed by the line 3. As the resistances R and R2 are simultaneouslyincreased the curve is moved tothe left as a result of the higher frequencies-being choked out-' oi the type having a plurality of electron tubes,

by-the inductance L and the curve is moved up as a result of the increased gain eilected by the increase of the resistance R2. As the areas under the curves afiord a measure of the output volume, it is clear from Fig. 3, that as the point of frequency cutoff is moved back, the volume may be held approximately constant.

As an example of constants which have been found to be suitablefin a circuit such as shown in 'F'ig.v Land in which the transformer 2was of a standard commercial design having a relatively fiat transmission characteristic; the following data is givenfor purposes of illustration, but the invention is not to be'understood as limited to these particular values:- L=10-15 henries I R=resistance variable from 0-500,000 ohms R1='resistance variable from 0-100,000 ohms While the invention hasbeen described in connection with an audion amplifier, it will be apparent that the tone control is equallyapplicable to other forms of audio frequency transmission circuits- Thesimultaneous control of amplifier gainand of the upper limit of the transmission band may be effected in other ways. For example,;in a radio receiver in which the amplification is regulated by the adjustment of a control grid bias, the bias controland resistance R may be coupled mechanically for simultaneous operation. 7 These and other variations maybe made without departing from 'the spirit ofmy invention asset forth in the following claims. 7

Iclaim:- l. The combination with an impedance adapted to couple two-sections of an audio fre quency transmission-line, ofa composite im-. pedance for controlling thetransmission-frequency characteristic of said: impedance and line, said composite impedance being iii-series with one section of said line and, comprising a fixed inductance and variable resistor shunted by anadjustable resistance;

2. As a tone control-for an audio frequency transmission line, the. combination with an audio frequency transformer for couplingtwo sections of said line, of an inductance and va- 7 resistance With means for adjusting the width of the band of frequencies which may be transmitted, of means for simultaneously therewith adjusting the gain of said amplifier in a direction opposite to the variation of band width.

5. In. an audio frequency amplifier circuit adapted to transmit a band of audio frequencies, the combination with means for adjusting the width of the band of frequencies which may be transmitted, with control means for simultaneously therewith adjusting the gain of said amplifier, said gain control means being arranged to increase the gain, as said first means decreases the width of the transmitted band.

6. An audio frequency transmission system said tubes having input and output circuits, and means for tone control, said means including an inductance and resistance in series with one of said output circuits, andmeans providing a path of adjustable impedance in shunt to said inductance.

7. A transmission system of the type having a plurality of electron tubes, said tubes having input and output circuits, and means for tone control including an inductance and a variable resistance having a maximum impedance greater than that of the inductance in parallel with each other and in series with one of said output circuits, and a variable resistor arranged for operation with the variable resistance.

' 8. A transmission system of the type having a plurality of electron tubes, said tubes having input and outputcircuits, means for tone control including a combination comprising an inductance and resistance in series with each other and in parallel with another resistance, said combination beingin series with one oi' said output circuits.

9. An audio frequency transmission system of the type having a pluralityof electron tubes, said electron tubes having input and output circuits, means coupling the output circuit of one of said tubes with the input circuit of another one of said tubes, means for tone control'in' series with said output circuit, and compensating means in "parallel with said output circuit to increase the amplification of said system, and means for simultaneously operating the compensating means with the adjustment of said control means in that sense which reduces the width: ofthe. band of transmitted frequencies? v 1" '10. In combination. in an audio frequency transmission" network, 'a'tube adapted to have its input electrodes connected to a source of audio'frequency modulated electrical waves, a succeeding. amplifier tube, an audio frequency transformer having its secondary connected to thefamplifier; input electrodes, means for controlling the transmission-frequency characteristic of the network connecting the output electrodes ofithe first, tube. to the transformer primary, said means" comprising a, coil connected between the .anodeof the first tube and, one side of the transformer primary, and a variable resister in shunt with the coil, the maximum impedance of the resistor exceeding that of the coil wherebywhen the resistor is variedto its maximum value the coil alone determines what high audio frequencies are transmitted to the said amplifier-tube.

11-. In combination in an audio frequency transmission network, a tubeadapted to have its input-electrodesconnected-to a source of audio frequency modulated electrical waves, a

audio frequency modulated electrical waves, a succeeding amplifier tube, an audio frequency transformer having its secondary connected to the amplifier input electrodes, means for controlling the transmission-frequency characteristic of the network connecting the output electrodes of the first tube to the transformerprimary, said means comprising a coil connected between the anode of the first tube and one side of the transformer primary, and a variable resistor in shunt with the coil, the maximum impedance of the resistor exceeding that of the coil whereby when the resistor is varied to its maximum value the coil alone determines what high audio frequencies are transmitted to the said amplifier tube and a variable resistor in series with the coil for regulating the sharpness of high audio frequency cut-ofl? by said controlling means.

12.111 combination in an audio frequency transmission network, a tube adapted to have its input electrodes connected to a source of succeeding amplifier tube, an audio frequency transformer having its secondary connected to the amplifier input electrodes, means for controlling the transmission-frequency characteristic'of the network connecting the output electrodes of the first tube to the transformer primary, said means comprising a coil connected between the anode of the first tube and one side of the transformer primary, andva variable resistor in shunt with the coil, the maximum impedance of the resistor exceeding that of the coil whereby when the resistor is varied to its maximum value the coil alone determines what high audio frequencies are transmitted to the said amplifier tube, and an adjustable resistor, operative simultaneously with said variable resistor, connected to the transformer to compensate for the loss in the output of said amplifier tube when said variable resistor is varied toward said maximum value.

STEPHEN D. LAVOIE. 

